I read Dr. Gordon Christensen’s June JADA article, "The Curing Light Dilemma," with great interest. I agree that there is confusion among practitioners regarding the optimal light-curing technique to utilize in their restorative procedures (high energy, low energy, ramped, stepped, pulsed, light-emitting diode and so forth).
Studies show that when the total energy (joules) delivered into a composite is the same—whether quickly for short periods or slowly over longer periods—the physical properties of the polymerized composite are the same.1,2 This holds true for clinically relevant exposure times and power densities. The total energy delivered is determined by multiplying exposure time by the power density of the light source (milliwatts per square centimeter).
The relevant question is whether direct composite restoratives can be polymerized quickly, with high-energy light sources, without untoward effects on materials and/or on biophysical properties of the teeth. If the answer is yes, then it would seem to make sense to use the fastest means possible to initiate and complete polymerization since it saves us time. If composite materials did not shrink, and assuming the device delivering the initiating energy does not generate potentially harmful heat, then there would be no controversy. Faster would be better.
But composite restorative materials do shrink (generally 2 percent to 5 percent by volume), and some devices do generate considerable heat; herein lies the controversy. Furthermore, the wavelength of the light delivered, the specific wavelength sensitivity of the photoinitiators, monomer reaction speed, modulus of the composite restorative and other variables also must be taken into consideration.
I disagree with Dr. Christensen’s assertion that "current clinical usage, as well as research, have disproved the allegations of damage caused by the faster lights." In fact, it all depends on whose research you look at. With all due respect to Dr. Christensen, he cites only one reference (his own Clinical Research Associates study3) to support his contention that high-intensity lights do not have an adverse effect on the integrity of bonded composite restorations and/or tooth tissues.
While there is equivocation in the literature, the majority of studies I’ve read (a sampling is cited here4–12) and presentations at the most recent International Association for Dental Research/American Association for Dental Research meeting,13,14 indicate clear advantages when utilizing lower-energy/longer-duration exposure times in terms of reducing shrinkage stress, marginal integrity and gap formation (at least in the laboratory).
I also think it’s irresponsible and misleading to suggest to clinicians that about three seconds is all that is required to cure composites with high-intensity light sources such as plasma arc curing, or PAC, lights. The fact is, there are a number of composites (and some adhesive systems) that will not cure with high-intensity light sources like PAC lights and argon lasers due to photoinitiator and light wavelength incompatibility.
Even when they are compatible, many studies have shown that three seconds of light curing is inadequate for many composite restoratives.15–17 Also, what if the light tip is a significant distance (as is often the case) from the surface of the composite during polymerization? Is three seconds enough in this scenario? One thing I know for sure is that composites cannot be overpolymerized, but they can definitely be underpolymerized.
What no one knows, including Dr. Christensen, is what the long-term clinical ramifications of high-intensity/short-duration polymerization will be. I am unaware of any long-term clinical studies that address this issue.
However, I do agree that anecdotal evidence in the short-term seems to support the use of high-intensity lights in certain situations. I have found, with some exceptions, that those using high-intensity lights are generally happy with the systems. But what will their restorations look like in five or 10 years? Dentistry is replete with techniques and materials that looked favorable initially, only to demonstrate significant problems and unacceptable failure rates as time went by (for example, all-ceramic bridges).
My intent here is not to impugn the beliefs of Dr. Christensen or refute his cited Clinical Research Associates study. I simply wish to make clear that the jury is still out on this issue. In fact, I know very intelligent and reasonable individuals who argue quite passionately and convincingly on both sides of this controversy. The objective individual recognizes and acknowledges equivocation when it exists despite his personal preference or beliefs.
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